Disclosed in Japanese Patent Application Laid-Open No. 2-169270 is an optical printer apparatus in which an optical head is moved relative to a sensitized sheet to form an image on the sensitized sheet. This optical printer apparatus will now be described with reference to FIG. 9.
A sensitized sheet 60 is driven at a constant speed in the direction of arrow A with respect to the optical head 10 by means of feed rollers 70. The optical head 10 comprises a white light source 20 for radially emitting white light, a cylindrical lens 30 for linearly converging the white light on the sensitized sheet 60, a three-color separation liquid crystal shutter 40, and a liquid crystal shutter 50.
The three-color separation liquid crystal shutter 40 is composed of three shutters 40r, 40g and 40b that linearly extend in the width direction (spreading direction) of the white light from the cylindrical lens 30. These three shutters 40r, 40g and 40b are driven independently of one another, and are provided individually with color filters that transmit red (R), green (G), and blue (B) light beams, respectively.
The liquid crystal shutter 50 includes a plurality of pixels that are arranged in the same direction as the lengthwise direction of the shutters 40r, 40g and 40b.
The following is a description of a method for forming an image on the sensitized sheet 60 by means of the apparatus shown in FIG. 9.
The optical printer apparatus receives gradated color image data, controls the shutters 40r, 40g and 40b in accordance with the image data, and exposes the surface of the sensitized sheet 60, thereby forming the image thereon. First, the shutter 40r opens for a given time to transmit the white light; next, the shutter 40g opens for the given time to transmit the white light; and then, the shutter 40b opens for the given time to transmit the white light.
This given time section is just equal to a period of time during which the sensitized sheet 60 moves for a distance X in FIG. 9.
Thus, the sensitized sheet 60 is exposed to the red light beam (R), which is first transmitted through the shutter 40r, for the distance X in its moving direction (direction A). Then, the shutter 40r is closed, while the shutter 40g opens. Since the sensitized sheet 60 is moved for the distance X by this time, that portion of the sensitized sheet 60 which is already exposed to the light beam R is exposed again to the green light beam (G) that is transmitted through the shutter 40g. When the sensitized sheet 60 further moves for the distance X, thereafter, the portion already exposed to the light beams R and G is exposed again in like manner to the blue light beam (B) that is transmitted through the shutter 40b. An image of full-color display can be obtained by repeating these processes of operation in the feeding direction of the sensitized sheet 60.
In a direction perpendicular to the feeding direction of the sensitized sheet 60, an image is formed by means of the liquid crystal shutter 50.
Referring now to FIG. 10, there will be described further in detail a method for gradation control by means of the printer apparatus shown in FIG. 9.
FIG. 10 is a diagram showing the relation between exposure time and position within the pixel width. For ease of illustration, in this case, the liquid crystal shutter 50 (and the optical head) is supposed to move in the z-direction with the sensitized sheet 60 kept stationary.
Let it be supposed that the light beam R with a width W, having passed through the liquid crystal shutter 50, is further moved at uniform speed in the z-direction so that an image A2 is formed after an image A1 is formed on the sensitized sheet 60.
Thereupon, the region that is exposed all the time from the formation of the image A1 on the sensitized sheet 60 to the formation of the image A2 is a region E within the pixel width. In this case, an exposure time t1 has a value obtained by dividing the distance for the movement of the image A1 to the image A2 (which is equal to the distance given by D and F in FIG. 10) by the moving speed of the optical head. In the regions D and F, moreover, the change of the exposure time, compared to the change of the z-direction position, is a linear increase or decrease. Thus, the relation between the position on the sensitized sheet 60 and the exposure time can be represented by a trapezoid B, as indicated by full line in FIG. 10.
When the light beam R with the width W, having passed through the liquid crystal shutter 50, is further moved in the z-direction so that an image A3 is formed after the image A1 is formed on the sensitized sheet 60, the relation between the position on the sensitized sheet 60 and the exposure time can be represented by a trapezoid C, as indicated by broken line in FIG. 10, if the distance for the movement of the image A1 to the image A3 is D/2.
The region that is exposed all the time from the formation of the image A1 on the sensitized sheet 60 to the formation of the image A3 corresponds to the top side of the trapezoid C indicated by broken line. In this case, the exposure time is t2 (=t1/2).
In the case where the gradation control is effected in a manner such that the light beam with the width W having passed through the liquid crystal shutter 50 is further moved in the z-direction to form the image A2 or A3 (or any other image) after forming the image A1 on the sensitized sheet 60, as described above, the trapezoid that is indicative of the relation between the position on the sensitized sheet 60 and the exposure time is always situated closer to one end (starting end) within the pixel width despite variation in height. Inevitably, therefore, there is a wide monochromatically exposed or unexposed portion in the region near the other end of the trapezoid. Accordingly, the image color mixture property is so poor that the image quality is lowered. Depending on the image pattern, moreover, interference fringes may be produced to make the image unclear in many cases.
The following problem is aroused, however, if the gradation control is effected in a manner such that the sensitized sheet 60 is exposed only for a time (t1, t2, etc.) corresponding to the given distance X by moving the image A1 on the sensitized sheet at uniform speed for the distance X in the scanning direction, as described above.
In general, the photosensitive properties of the sensitized sheet 60 with respect to time are non-linear. More specifically, the sensitization speed for the sensitized sheet is not always constant without regard to the exposure time. As the exposure time becomes longer, the sensitization speed lowers gradually.
Let it be supposed that the maximum exposure time is divided into N number of equal parts (N: integer), and image data i are made to correspond to [maximum exposure time.times.(i/N)] (i: an integer not larger than N). If the gradation is controlled by means of the image data i, the image data cannot be reproduced with correct gradation.
In general, moreover, the sensitization speed of the sensitized sheet 60 varies also depending on the waveform of light. Thus, in a color image, the gradation inevitably delicately varies with color without any change in exposure time.